1. Field of the Invention
The invention relates to a so-called dielectric barrier discharge lamp in which excimer molecules are formed by a dielectric barrier discharge, and in which the light emitted from the excimer molecules is used, for example, as an ultraviolet light source for a photochemical reaction.
2. Description of the Related Art
From unpublished Japanese patent specification HEI 1-144560 or U.S. Pat. No. 4,837,484, a radiator, i.e., a dielectric barrier discharge lamp, is known as generic technology, in which a discharge vessel is filled with a gas which forms an excimer molecule, and in which light which is emitted by a dielectric barrier discharge from the excimer molecules is emitted from the lamp.
This dielectric barrier discharge is also called an ozone production discharge or a silent discharge, as is described in the "Discharge Handbook", Elektrogesellschaft, June 1989, 7th edition, page 263.
In the aforementioned publication, it is described that a transparent discharge vessel which has a roughly cylindrical shape works at least partially also as the dielectric of the dielectric barrier discharge, and in it the light is emitted from excimer molecules. Furthermore it is described that an outside tube and an inside tube are arranged coaxially to one another as a double tube, that the outside surface of the outside tube is provided with a network-like electrode, that the inside surface of the inside tube is provided with an inside electrode, and that in the discharge space, between this outside tube and this inside tube, the dielectric barrier discharge is accomplished.
A dielectric barrier discharge lamp of this type has advantages which neither a conventional mercury low pressure lamp nor a conventional high pressure arc discharge lamp have; for example, the emission of ultraviolet beams with short waves, in which the primary wavelengths are 172 nm, 222 nm, and 308 nm, and at the same time selective generation of light with individual wavelengths with high efficiency which are roughly like line spectra can be achieved.
Furthermore, this type of lamp has the advantage that commercial quartz glass can be used for the discharge vessel, a simple arrangement of the entire lamp can be obtained and production can be easily achieved if it has a roughly cylindrical outside shape and a coaxial arrangement of the outside tube and the inside tube, as is described above.
The conventional dielectric barrier discharge lamp however has the following disadvantages:
(1) First of all, the inside electrode cannot be easily manufactured.
In order to effectively supply power to the inside electrode to cause the dielectric barrier discharge, it is necessary to arrange the inside electrode head-to-head tightly against the inside tube. Conventionally, therefore, an electrode in the form of a thin film was formed in the inside tube by a vapor deposition process.
The inside tube, however, has, for example, a diameter from 10 to 20 mm and a length of roughly 100 mm to 1000 mm. This means that vapor deposition must be performed within this narrow space and formation of a vapor deposited film with a uniform thickness was not possible.
Furthermore, the film formed by vapor deposition detaches easily from the inside tube if its thickness is greater than or equal to 0.01 mm.
Moreover, the disadvantage arose that nondestructive study of the thickness of the inside electrode is not possible even if the inside electrode can be advantageously formed.
(2) Second, the inside electrode easily corrodes if the dielectric barrier discharge lamp is operated over a long time. Corrosion occurs especially easily in a part in which its thickness is low. If corrosion causes a decrease of conductivity, it no longer functions as an electrode, and the service life of the lamp is shortened. The mechanism for this corrosion of the inside electrode presumably functions as follows:
(a) For the discharge gas encapsulated in the discharge space, a mixed gas of chlorine with an inert gas, such as xenon or argon or the like, is used. The action of these gases causes emission of vacuum ultraviolet light which is absorbed by the oxygen, generating ozone therefrom. PA1 (b) In the dielectric barrier discharge lamp of the double tube type which consists of an inside tube and an outside tube, for safety, the inside electrode located in the inside tube is located on the high voltage side and the network-like electrode located in the outside tube is located on the ground side. The reason for this is that there is only a small probability that the inside electrode will come into contact with individuals and the like.
Furthermore, there are cases in which, in the inside electrode to which high voltage is applied, a glow discharge occurs which also produces ozone from the oxygen.
As described above, it is possible for the inside electrode to be corroded by the ozone which is produced by the vacuum ultraviolet light and the glow discharge.
On the other hand, it is possible in a conventional discharge lamp to provide a large interval between the electrode and the line and the light emission part in which the ozone is generated. In the dielectric barrier discharge lamp, however, it is fundamentally impossible to effect a large interval between the electrode and the light emission part by the arrangement in which the electrode and the light emission part are arranged adjacent to one another.
This means that the above described disadvantage of corrosion by ozone is characteristic of a dielectric barrier discharge lamp.
It is certainly conceivable that instead of producing the above described inside electrode by the vapor deposition process, a metal tube is used as the inside electrode. This process can eliminate the disadvantage of the above described vapor deposition process. However, the disadvantage arises that the adhesive property between the inside electrode and the inside tube deteriorates seriously, and that the amount of power supplied to the discharge space has a dispersion since the inside diameter of the commercial quartz glass tube which is used as the inside tube has a large tolerance of .+-.0.5 mm.